Perovskite materials have a general formula of ABX3, where A is an alkali metal, an alkali earth or a rare earth; B is a transitional metal, and X is generally oxygen; Bums et al., Space Groups for Solid State Scientists, 2d Ed., Academic Press (1990).
In recent years there has been an explosion of interest in perovskite manganite materials, R1-xAxMnO3, where R is one or more rare earths, such as lanthanum or praseodymium, and A is an alkali or alkaline earth, such as calcium, strontium or barium. This is primarily due to the discovery of colossal magnetoresistance (CMR), in which the resistance can be changed by several orders of magnitude by application of a large magnetic field. This effect occurs because the applied magnetic field drives a phase transition from an insulating paramagnetic state to a spin-aligned metallic state, Mathur et al., Mesoscopic Texture in Manganites, Physics Today, p. 25-30, January 2003. Because the valence electrons in some cases are fully spin-polarized, these materials may be usable in future spin-injection and spin-tunneling devices. However, these effects require cryogenic temperatures.
Another reason for the increase in interest in manganites is the discovery of a room temperature effect in which the resistance can be changed from a high to a low state via a voltage pulse, which could be utilized in high density non-volatile memories, Zhuang et al., Novell [sic] Colossal Magnetoresistive Thin Film Nonvolatile Resistance Random Access Memory (RRAM), Technical Digest of the 2002 International Electron Devices Meeting, p. 193, (2002).
Providing high-quality epitaxial manganite films may be of great use in both these types of applications, however, this requires a lattice-matched substrate. Fortunately, many of these materials have lattice constants very close to that of silicon. For example, Pr1-xCaxMnO3, (Pbnm space group, orthorhombic) has lattice constant a=5.426 Å, b=5.478 Å, and c=7.679 Å for x=0.3. Lattice constants “a” and “b” are just −0.09% and +0.86% different from the silicon lattice constant of 5.431 Å, respectively, Jirak et al., Neutron Diffraction Study of Pr1-xCaxMnO3 Perovskites J. of Magnetism and Magnetic Materials, vol. 53, 153 (1985). A related perovskite, SrTiO3, has a cubic lattice constant of 3.905 Å. This is only 1.7% larger than the Si(110) distance of 3.840 Å, so growth on Si(100) may be accomplished with a 45° rotation of the perovskite lattice.
Consequently, numerous researchers have proposed ways to deposit nominally single crystal rare-earth oxides and perovskite oxides, not containing manganese, epitaxially on silicon for CMR, as well as for ferroelectric memory, micro-electro-mechanical system (MEMS), ferromagnetic, piezoelectric, pyroelectric, electro-optic or large dielectric constant applications, U.S. Pat. No. 6,642,539, to Ramesh et al., granted Nov. 4, 2003, for Epitaxial template and barrier for the integration of functional thin film metal oxide heterostructures on silicon; U.S. Pat. No. 6,518,609, to Ramesh, granted Feb. 11, 2003, for Niobium or vanadium substituted strontium titanate barrier intermediate a silicon underlayer and a functional metal oxide film; U.S. Pat. No. 6,432,546 to Ramesh et al., granted Aug. 13, 2002, for Microelectronic piezoelectric structure and method of forming the same; Liu et al., Epitaxial La-doped SrTiO3 on silicon: A conductive template for epitaxial ferroelectrics on silicon, App Phy Let, vol. 80, no. 25, pp. 4801-4803, (2002); U.S. Pat. No. 6,610,548, to Ami et al., granted Aug. 26, 2003, for Crystal growth method of oxide, cerium oxide, promethium oxide, multi-layered structure of oxides, manufacturing method of field effect transistor, manufacturing method of ferroelectric non-volatile memory and ferroelectric non-volatile memory; U.S. Pat. No. 5,225,031, to McKee et al., granted Jul. 6, 1993, for Process for depositing an oxide epitaxially onto a silicon substrate and structures prepared with the process; U.S. Pat. No. 5,830,270, to McKee et al., granted Nov. 3, 1998, for CaTiO3 Interfacial template structure on semiconductor-based material and the growth of electroceramic thin-films in the perovskite class; McKee et al., Crystalline Oxides on Silicon: The First Five Monolayers, Physical Review Letters, vol. 81, no. 14, pp. 3014-3017 (1998); and Hu et al., The interface of epitaxial SrTiO3 on silicon: In situ and ex situ studies, App Phy Let, vol. 82, 203 (2003). The deposition method is usually ultra-high vacuum molecular-beam epitaxy (UHV-MBE), at least to grow the initial atomic layers of the epitaxial film. For example, SrTiO3 has been successfully deposited epitaxially on Si(100) using UHV-MBE with a carefully controlled deposition sequence to avoid formation of an interfacial SiO2 layer, which would destroy the template for epitaxy, Hu et al., supra. Specifically, Hu et al. deposited a few monolayers of Sr on a silicon surface covered with native oxide. During heating, up to 750° C. to 850° C., the strontium reduced the native oxide, leaving only ordered strontium on the silicon surface, a Sr/Si(2×1) reconstruction. SrTiO3 was then grown by co-deposition of strontium and titanium metals in an oxygen ambient with very low partial pressure (10−8 torr to 10−7 torr), with the substrate at 300° C. to 400° C. If the substrate was heated to only 500° C., oxygen diffused through the film and led to formation of an SiO2 layer at the SrTiO3/Si interface, which is undesirable. U.S. Pat. No. 5,830,270 describes epitaxial growth on silicon-germanium, as well as silicon, specifically adjusting the composition of the epitaxial oxide to minimize strain on the SiGe lattice.
Rare earth binary and ternary oxides, such as (LaxY1-x)2O3 have been epitaxially deposited on Si(111) substrates, Guha et al., Lattice-matched, epitaxial, silicon-insulating lanthanum yttrium oxide heterostructures, App Phy Let, vol. 80, 766 (2002); and Narayanan et al., Interfacial oxide formation and oxygen diffusion in rare earth oxide-silicon epitaxial heterostructures, App Phy Let, vol. 81, 4183 (2002). Diffusion of oxygen through the film during the high temperature growth was noted. It was also noted that, when using MBE, metal-first deposition resulted in silicidation of the surface, causing a rough oxide film nucleation. However, using oxygen-first deposition, smooth epitaxial films were formed, as evidenced by reflection high energy electron diffraction (RHEED) and x-ray diffraction (XRD), Guha et al., supra. So, in some cases oxygen-first deposition seems to work better than metal-first deposition.
Another active area of research has been the effect of strain on CMR properties, induced, for example, by deposition on different substrates, Wu et al., Substrate induced strain effects in epitaxial La0.67-xPrxCa0.33Mn03 thin films, Journal of Applied Physics, vol. 93, 5507 (2003). In some cases the effects may be dramatic and the tuning delicate. A well-controlled way to adjust the strain is desired. For example, Wu, et al. studied the effect of strain on La0.67-xPrxCa0.33MnO3 (LPCMO), with x=0.13 to 0.27. To adjust the strain from tensile to near zero to compressive, Wu, et al. had to change the substrates, from SrTiO3 to NdGaO3 to LaAlO3. Wu, et al. found metastable phase mixtures in the films, with the volume fractions controlled by strain. In their samples they found that large tensile strain eliminated metallic behavior. Also, they found the magnetoresistance of LPCMO at low field is increased for films under tensile strain. They conclude: “Controlling strain is the first step of constructing any successful devices based on manganite thin films.”
Others have found that the metal-insulator transition can be suppressed in very thin CMR films because of strain, Jin et al., Thickness dependence on magnetoresistance in La—Ca—Mn—O epitaxial films, App Phy Let, vol. 67, 557, (1995). Theory suggests that the ferromagnetic Tc is extremely sensitive to biaxial strain which can affect both the Mn—O bond distance and the Mn—O—Mn bond angle, Millis et al., Quantifying strain dependence in “colossal” magnetoresistance manganites, J. Appl. Phys., vol. 83, 1588 (1998). It has been reported that thin films of LSMO are metallic under tensile strain and insulating under compressive strain, Konishi et al., Orbital-State-Mediated Phase-Control of Manganites, J. Phys. Soc. Jpn. vol. 68, No. 12, 3790 (1999).
In another example, during epitaxial growth of (LaxY1-x)2O3 on Si(111), Guha, et al., supra, changed the lattice matching by adjusting the La:Y ratio. To minimize the strain, they also had to take thermal expansion into consideration. However, it would be desirable to minimize strain by adjusting the lattice constant of the substrate, instead of changing the composition of the epitaxial oxide, which could then change the properties of that oxide. It would also be desirable to change the strain in the substrate without having to change the type of substrate.
A different area of research has been the production of so-called “virtual substrates”, in which relaxed SiGe films are fabricated on silicon substrates. The current predominant technique to produce a high quality relaxed Si1-xGex virtual substrate is the growth of a several μm thick compositionally graded layer, Watson et al., Relaxed, low threading defect density Si0.7Ge0.3 epitaxial layers grown on Si by rapid thermal chemical vapor deposition, J. Appl. Phys., vol. 75, 263 (1994); and Currie et al., Controlling threading dislocation densities in Ge on Si using graded SiGe layers and chemical-mechanical polishing, App Phy Let, vol. 72, 1718 (1998), wherein the density of threading dislocations is typically ˜106/cm2.
Recently, alternative methods to efficiently relax thinner, e.g., 100 nm to 500 nm, strained SiGe layers on silicon have been sought. Atomic hydrogen (H+) implantation followed by an appropriate anneal, e.g., 800° C. for several minutes, has been used to increase the degree of SiGe relaxation and to reduce the density of threading dislocations, Mantl et al., Strain relaxation of epitaxial SiGe layers on Si (100) improved by hydrogen implantation, Nuclear Instruments and Methods in Physics Research B 147, 29, (1999); Trinkaus et al., Strain relaxation mechanism for hydrogen-implanted Si1-xGex/Si (100) heterostructures, App Phy Let, vol. 76, 24, 3552 (2000); U.S. Pat. No. 6,464,780, to Mantl et al., granted Oct. 15, 2002, for Method for the Production of a Monocrystalline Layer on a Substrate with a Non-adapted Lattice and Component containing One or Several Such Layers; and U.S. Pat. No. 6,746,902, to Maa et al., granted Jun. 8, 2004, for Method to Form Relaxed SiGe Layer with High Ge Content. Previously, we have proposed the implantation of either H+ or H2+ alone or with boron, helium, silicon, or other species for the purpose of relaxing strained SiGe films deposited epitaxially on silicon substrates, U.S. patent application Ser. No. 10/936,400, of Tweet et al., filed Sep. 7, 2004, for Method to Form Relaxed SiGe Layer with High Ge Content using co-implantation of boron and hydrogen; and U.S. Pat. No. 6,562,703, to Maa et al., granted May 13, 2003, for Molecular hydrogen implantation method for forming a relaxed silicon germanium layer with high germanium content. Also, the implantation of He+ alone or Si+ alone have both been successful, Luysberg et al., Effect of helium ion implantation and annealing on the relaxation behavior of pseudomorphic Si1-xGex buffer layers on Si(100) substrates, J. Appl. Phys., vol. 92, pp 4290-4295 (2002); Cai et al., Strain relaxation and threading dislocation density in helium-implanted and annealed Si1-xGex/Si(100) heterostructures, J. Appl. Phys., vol. 95, pp 5347-5351 (2004); and Hollander et al., Strain relaxation of pseudomorphic Si1-xGex/Si(100) heterostructures after Si+ ion implantation, J. Appl. Phys., vol. 96, pp 1745-1747 (2004).
Another method to produce a relaxed, thin SiGe virtual substrate having a low density of threading dislocations has been to grow the SiGe on a buffer layer that has an abundance of point defects, such as silicon interstitials. This has been accomplished by growing a silicon, Linder et al., Reduction of dislocation density in mismatched SiGe/Si using a low-temperature Si buffer layer, Appl. Phys. Let., vol. 70, 3224 (1997), and Lee et al., Effects of low-temperature Si buffer layer thickness on the growth of SiGe by molecular beam epitaxy, J. Appl. Phys., vol. 92, 6880 (2002), or SiGe buffer layer at low temperatures or by implantation of Si+ ions during buffer growth, Kasper et al., New virtual substrate concept for vertical MOS transistors, Thin Solid Films 336, 319 (1998).